There are many reports in the prior art describing the aggregation of plant food particles to facilitate placement and release of nutrients in desired locations, and in some cases, at desired release rates.
In U.S. Pat. No. 3,150,955 the use of ureaformaldehyde polymers is reported as a source of slowly water soluble nitrogen for fertilizers. U.S. Pat. No. 3,502,458 teaches the use of a urea-formaldehyde thermoset resin as a binder in a fertilizer composition which contains fibrous particles, such as sawdust or peat moss, in addition to the sources of plant nutrients. The spike composition taught required a support member in the spike. U.S. Pat. No. 3,057,713 discloses a pointed stick of fertilizer which can be driven into the ground, made by binding together granulated sources of nitrogen, phosphate, and potassium with reaction product of fibrous asbestos and phosphoric acid. U.S. Pat. No. 3,647,416 discloses a cold chisel shaped product which does not employ resins but is formed by the extrusion of a composition containing a filler, such as petroleum coke, sand, clay, or coke breeze, and a binder formed by the reaction of inorganics magnesium oxide and phosphoric acid.
U.S. Pat. No. 4,348,218 discloses compression of a substantially homogeneous granular fertilizer mixture and a thermosetting binder composed of urea-formaldehyde resin to form a briquette, which is then cured into a drivable spike. The spike, when driven into the ground, fertilizes the immediate area at a slow, even rate. The urea-formaldehyde resin binder comprises between 5 and 15 percent by weight of the briquette.
U.S. Pat. No. 3,892,552 discloses a one-piece wedge-shaped spike of fertilizer source materials bound together by a cured thermoset resin, which functions best when a plastic cap is placed on top of it for hitting, while the spike is hammered into the ground. The thermoset resin cited in the disclosure is a urea-formaldehyde resin in a 10 percent by weight concentration.
The spikes of the prior art are all bound together by substantial amounts of hardened materials, such as asbestos fibers, petroleum cake, and thermoset resin. These hardened binders have limited resistance to attrition and shattering when they are hammered into the ground and require relatively large amounts of the binders to hold the spikes together.
The spikes of the prior art are not inherently controlled release materials and obtain some controlled release properties by the occlusion of soluble materials in the spike materials. Although urea formaldehyde polymers are known to be effective fertilizers, polymerization to a high strength thermoset plastic is well known to greatly reduce their effectiveness as fertilizers. Prior art spikes require substantial amounts of binding material, typically amounting to 10 percent or more.